1. Field of the Invention
The present invention relates to an apparatus for connection to a communication bus. More specifically, it relates to an apparatus for encoding the status of several emergency switches for communication across a so-called AS-interface.
2. Description of the Related Art
The Aktuator-Sensor-Interface, typically referred to as an AS-interface or AS-i, is a two-wire master-slave bus system as described in “AS-Interface, The Automation Solution” published 2002 by the AS-International Association, Zum Taubengarten 52, 63571 Gelnhausen, Germany, the full content of which is hereby incorporated by reference. The AS-interface has been standardized in European standard EN 50295, the full content of which is hereby incorporated by reference. An alternative version of the AS-interface has been standardized in IEC Standard 62026-2, the full content of which is hereby incorporated by reference. Bus systems compliant with any of the aforementioned references may be termed an AS-interface.
The two wires of an AS-interface are used for data communication and power supply. An AS-interface has a single master node (hereinafter also just “master”) and a plurality of slave nodes node (hereinafter also just “slave”). The master queries each of the slaves individually. In other words, only the queried slave responds. The master queries each of the slaves sequentially. In other words, the master queries each of the slaves, one after the other, in accordance with a predetermined sequence. Such a sequential querying of all slaves is designated as a cycle. Once all slaves have been sequentially queried by the master, the cycle is repeated. Between any two cycles, the master may exchange data with any one of the slaves. The master queries the slaves in accordance with a predetermined timing.
In the context of an AS-interface, the exchange of data between the master and a slave during a cycle is termed cyclic communication, and the exchange of data between the master and a slave between any two cycles is termed acyclic communication. Hereinafter, data communicated in an AS-interface by cyclic communication are referred to as data codes, and data communicated in an AS-interface by acyclic communication are referred to as parameters or parameter codes.
The AS-interface specification includes so-called “Safety at Work” provisions that allow both safety information and normal data to be communicated across the same AS-i cable. For example, the emergency wiring of a machine can be integrated into the control wiring of that machine without significant additional overhead, yet while complying with the requirements of Safety Integrity Level 3 (SIL3) of IEC standard 61508 and Category 4 of European standard EN 954-1.
To ensure secure querying of safety-relevant slaves that are connected to safety components such as emergency buttons and that supply corresponding safety signals, the (querying/request) data packets that are sent from the master to safety-relevant slaves as well as the (response) data packets that are sent from a safety-relevant slave to the master during cyclic communication include special bit patterns that are several bits in length. Specifically, a secure slave, i.e. a slave compliant with the AS-i “Safety at Work” provisions, codes safety relevant information, e.g. the information “emergency button not activated,” into a 32-bit long codeword, four respective bits of which are communicated per cycle over a consecutive sequence of eight cycles.
The use of 32-bit codewords theoretically allows for over four billion, namely 232, different codewords. However, the AS-i specification includes rules that restrict the available codewords to roughly 950,000 possibilities. The available codewords are centrally administered and individually issued to ensure that each secure AS-i slave is globally uniquely identifiable based on the “fingerprint” of its codeword(s).
In accordance with the AS-i “Safety at Work” provisions, a safety monitor monitors the data communicated between the master and any secure slaves during cyclic communication. If the safety monitor detects a codeword from a secure slave that, instead of the expected codeword, contains a zeroed bit pair at the beginning and/or the end of the four bits communicated in a respective cycle, the safety monitor activates the safety state associated with the respective secure slave. In the case of a secure slave that transmits information indicative of the state of an emergency button on a machine, for example, divergence from the expected codeword indicating that the emergency button has not been activated could trigger the safety monitor to cut off all power to that machine and to perhaps activate an emergency braking mechanism for that machine.
In practice, it is often desired to install safety-relevant devices, e.g. emergency buttons, door contact switches, etc., at various positions in an installation. Conventionally, networking these safety-relevant devices via an AS-interface requires a corresponding number of secure slaves, which is impractical. Alternatively, the safety-relevant devices can be connected in series. In latter case, however, when one of the serially connected, safety-relevant devices is activated, the safety monitor receives no information specifying which of the serially connected, safety-relevant devices has been activated. While this lack of specific information may be irrelevant for many safety aspects of an installation, e.g. for ensuring that the appropriate machine or that the overall installation is shut down, it is generally desirable to obtain such information e.g. for guiding firefighting or rescue crews or for allowing the “fault” to be pinpointed and remedied by maintenance personnel.
Similar problems can arise when networking numerous devices via other, i.e. non-AS-i, bus systems or when networking numerous non-safety-critical devices via an AS-interface.
It is an object of the present invention to overcome these deficiencies of the prior art.